Small Fossil Remembers When Continents Collided

When were the mountains of Wales pushed up?

It was well before the dinosaurs roamed the earth. And it happened in the aftermath of a gigantic continental collision, when England and Wales (then attached to southern Newfoundland) crashed into Scotland (then attached to north America). The muds of the sea floor were converted, then, into the hard grey slates of the Welsh hills.

Until now it has been very hard to tell exactly when these slates were formed, because the minerals that make up the slates have been just too tiny for scientists to be able to separate out and measure their age.

New research by paleontologist Jan Zalasiewicz, of the University of Leicester Geology Department, and colleagues from the Open University and the British Geological Survey is now able to shed light on this geological mystery – thanks to some small fossils.

Fossil graptolites – mysterious, extinct planktonic creatures – have been found preserved in the slates, their remains beautifully mineralized in shiny yellow iron pyrites (fool’s gold).

When the muds were crushed into slates, they were deformed around the hard pyritized graptolites. Little spaces opened between the fossils and the rock as this happened, and these spaces were filled with pure new slate-forming minerals.

These minerals were large enough and pure enough for Sarah Sherlock of the Open University to extract and to date, exploiting their natural radioactivity as a kind of natural clock.

The results show that the slates formed 396 million years ago (plus or minus 1.4 million years). This is by far the most precise date extracted from the slates of the Welsh hills, or indeed from slates anywhere in the world.

This pioneering method can now be applied to measure the ages of mountain belts everywhere, since fossils preserved in fool’s gold can be found all around the world.